1. Technical Field
The present invention relates to a device for interferential distance measurement.
2. Background Information
Besides the acquisition of position changes of two objects movable relative to one another in a lateral direction, there are also measurement tasks in which it is solely, or additionally, necessary to determine the distance between these objects in a vertical measuring direction, perpendicular to the lateral direction. For high-precision distance measurement in such a measuring direction, interferential methods such as those disclosed in DE 10 2007 016 774 A1 or DE 10 2010 003 157 A1 may be considered.
The device for interferential distance measurement known from DE 10 2007 016 774 A1 includes an emitter-receiver unit, which is disposed on a glass plate and placed at a distance to be determined from an object, and a mirror is disposed on the object. Splitter gratings that split the beams emitted by the light source into at least one measurement beam and at least one reference beam are disposed on the glass plate. The measurement beam is propagated in the direction of the mirror on the object and is reflected from it back in the direction of the emitter-receiver unit. The reference beam is propagated solely within the glass plate, and after multiple reflections it enters into interferential superposition with the measurement beam in the emitter-receiver unit. From the interference signals obtained in this way, the distance between these components can be ascertained. One disadvantage of this device is that in the event of tilting between the glass plate and the mirror, erroneous scanning signals result. Another disadvantage of this device is that the measurement outcome depends on the wavelength of the light source employed. The wavelength can vary because of fluctuations in ambient conditions and can thereby cause mistakes in the distance measurement.
The device known from DE 10 2010 003 157 A1 solves the aforementioned problems in DE 10 2007 016 774 A1 by appropriate beam guidance of the measurement and reference beams. At least at a predetermined set-point distance, it is ensured that the distance measurement is independent of any wavelength fluctuations and is not vulnerable to tilting.
A device for interferential distance measurement that by comparison is optimized still further is known from the publication “Non-contact displacement meter for splitter element resolution” by Hideaki Tamiya (Precision Engineering Society of Japan, Spring Meeting, March 2012). This device includes a measurement reflector, a light source, a splitter element in the form of a beam splitter cube, a combining element, and a detector arrangement. Via the splitter element, a beam emitted by the light source is split into at least one measurement beam and at least one reference beam. Further down the path of the beam, the measurement beam acts four times on the measurement reflector, before entering at the combining element into interferential superposition with the reference beam. Via the detector arrangement, at least one scanning signal is generated from the interfering measurement and reference beams, relating to the distance in the measuring direction between the measurement reflector and one or more other components of the device.
A disadvantage of that proposed device is that in the event of a deviation in the actual wavelength from an assumed nominal wavelength if the measurement reflector tilts, the result is a measurement error in the distance determination. In this regard, see FIG. 1, which shows a fragmentary view of the beam path of the device from the aforementioned publication. From top left in this view, the measurement beam M strikes the measurement reflector MR at the impact site A1 at an angle of incidence α=45°, then reaches a grating G and next strikes the measurement reflector MR again at the impact site A2. After a rereflection, not shown, at a retroreflector, the measurement beam M takes the same path a second time in the opposite direction and acts on the measurement reflector MR a total of four times before it enters into interferential superposition with the reference beam—not shown. The interference signal that thus results represents the scanning signal to be determined, in the event of changes in the distance by which the measurement reflector MR is spaced apart from the remaining components in the measuring direction z shown in FIG. 1.
In the event of tilting of the measurement reflector MR about the y axis indicated, an observation of the k vector of the measurement beam M in the course of the distance traveled furnishes a resultant phase shift φk in the measurement beam M, in accordance with the following equation (1):φk=8·√2·π·Ry·Δx·(1/λ0−1/λ)  (equation 1)
in which                φk=phase shift upon tilting of the measurement reflector MR about the y axis; 4 interactions of the measurement beam with the measurement reflector; α=45°        Ry=angle of rotation about the y axis        Δx=distance of the impact sites A1, A2 from the tilt axis in the x direction        λ=actual wavelength        λ0=nominal wavelength.        
As can be seen from equation (1), upon such tilting and a deviation of the actual wavelength λ from the nominal wavelength λ0, there is a resultant phase shift φk≠0 on the part of the measurement beam M. Such a phase shift φk arises from the wavelength-dependent deflection at the grating G and the attendant displacement of the impact site A2 in the case where λ≠λ0. In the scanning signal generated, it causes a change in the spacing distance, even though nothing has changed with regard to the distance to be measured in the measuring direction z. In the case of the parameters Ry=5 mrad, Δx=5 mm, λ0=780 nm, and λ=λ0+5 nm, the result with equation 1 would be a phase shift φk=1.15·2π in the measurement beam M, which causes a considerable error in the distance determination.
Accordingly, the proposed device in the aforementioned publication is not independent, under all conditions, of possible resultant changes in wavelength. Such changes can for instance be due to changing ambient, conditions and if tilting of the measurement reflector occurs, they cause erroneous measurements of the distance to be determined.